But an influx of more than $1.5 million in outside spending on ads attacking Summers’ primary opponent, front-runner Angus King, means the loudest Republican voices Maine voters have heard this election season aren’t from the campaign those ads are designed to benefit.

“When you have such a flood of money coming in, what it basically means is that the outside groups are responsible for most of the information that voters hear,” said Anthony Corrado, a political science professor at Colby College in Waterville who has researched the role of money in politics.

And that means the dynamics have changed for the traditional candidate campaign operation this election season, especially because the amount of money outside groups are spending exceeds the $1.2 million King, Summers and Dill had raised in total by June 30, the end of the most recent financial reporting period.

The National Republican Senatorial Committee, the GOP’s Senate campaign arm, has spent almost $360,000 so far on television and radio spots that attack King and Dill, according to spending reports the group has filed with the Federal Election Commission. The organization is expected to spend $650,000 on the Maine Senate race this month, and it uploaded the second ad in its campaign — another spot that targets King on wind power — to its YouTube channel Friday.

Maine Freedom, a recently formed group whose officers have Republican ties, has spent nearly $360,000 on its advertising campaign urging Democrats to choose Dill over King.

And the U.S. Chamber of Commerce has spent $839,000 on two ads attacking King for increased state spending during his two terms as governor.

“They’re outspending the candidates, and that puts more pressure on the candidates’ campaigns to raise money in response,” Corrado said.

“A candidate now has to take on two tasks,” Corrado said. “One is making the case for why they should be elected. The other is the need to respond to criticism waged by the ads by these outside groups. The candidate in some ways has less control of the issues that are going to become the primary focus of voter attention.”

With polls showing King’s lead shrinking — at least in part due to the outside negative ads — the former governor’s campaign appeared to start shifting gears this week. In an interview with WCSH-TV 6, King said, “We are going to come back and draw the distinctions between myself and Charlie Summers.”

The campaign recently issued the first of what it called a “daily roundup of mistruths, accusations and now personal attacks,” and started publicizing videos about the state budget process that rebut claims the U.S. Chamber of Commerce is making about state spending under King. The campaign also has devoted some of its television advertising recently to addressing outside spending.

“We have always said we will defend ourselves and point out the differences between the candidates,” King said through a spokeswoman. “Mentioning our opponents by name and pointing out those differences is not mudslinging.”

For Summers, the candidate who stands to benefit most from the outside spending on ads, the effect is different.

“Summers can look a little bit like he’s above the fray by talking about himself,” said Jim Melcher, a political science professor at the University of Maine at Farmington. “In some ways, it creates an opening for him to talk about himself.”

The outside ad spending also narrows the fundraising gap between the Summers and King campaigns, said Corrado. The latest reports on file with the Federal Election Commission showed King had raised $897,000 for his campaign by June 30 to Summers’ $239,000.

“It relieves some of the resource demands on the campaign,” Corrado said. “It’s still the case that the candidate has to perform well on the campaign trail and make the case why he should be elected.”

“Charlie will continue meeting with the farmers, truck drivers and factory workers across Maine who know only a former small businessman has the wherewithal to balance budgets, cut taxes and end runaway spending in Washington,” he said in an email.

And on election day, outside groups launching television ads are no substitute for an on-the-ground campaign operation working to get voters to the polls, according to Corrado.

Outside groups “tend to focus on campaign advertising because they don’t have big memberships here in the state,” he said. “They don’t have a membership that they can mobilize to go out and vote.”